A. Neural Stimulation Treatments
Existing patient treatments include applying stimulation (e.g., up-regulating) signals to nerves, muscles or organs for treating a wide variety of medical disorders. Stimulation signal parameters (e.g., pulse width, frequency, and amplitude) are selected to initiate neural action potentials to be propagated along the nerve to an organ (e.g., brain or stomach).
Down-regulating signals also can be applied to nerve fibers. Certain signal parameters can result in a signal that inhibits the nerve or blocks the propagation of action potentials along the nerve. In general, the nerve conduction block is applied to nerves with down-regulating signals selected to block the entire cross-section or part of the cross section of the nerves (e.g., afferent, efferent, myelinated, and non-myelinated fibers) at the site where the down-regulating signal is applied.
In some systems, down-regulating signals are used to manage motor control over certain areas of a patient's body. For example, cryogenic nerve blocking of the vagus nerve to control motor activity is described in Dapoigny et al., “Vagal influence on colonic motor activity in conscious nonhuman primates,” Am. J. Physiol., 262: G231-G236 (1992). A cryogenic vagal block and the resulting effect on gastric emptying are described in Paterson C A, et al., “Determinants of Occurrence and Volume of Transpyloric Flow During Gastric Emptying of Liquids in Dogs: Importance of Vagal Input,” Dig Dis Sci, (2000); 45:1509-1516.
B. Application to Chronic Pain
Applying up-regulating electrical energy to the spinal cord for the purpose of managing pain has been actively practiced since the 1960s. While a precise understanding of the interaction between the applied electrical energy and the nervous tissue is not fully appreciated, it is known that application of an electrical field to spinal nervous tissue can effectively mask certain types of pain transmitted from regions of the body associated with the stimulated tissue. Such spinal cord stimulation (SCS) for the treatment of chronic intractable pain was introduced by Shealy et al. (Anesth. Analg., 46, 489-491, 1967).
More specifically, applying up-regulating electrical pulses to the spinal cord associated with regions of the body (e.g., dermatomes) afflicted with chronic pain can induce paresthesia, or a subjective sensation of numbness or tingling, in the afflicted bodily regions. This paresthesia can effectively mask the non-acute pain sensations perceived at the brain.
Electrical energy, similar to that used to inhibit pain perception, also may be used to manage the symptoms of various motor disorders, for example, tremor, dystonia, spasticity, and the like. Motor spinal nervous tissue (e.g., nervous tissue from ventral nerve roots) transmits muscle/motor control signals. Sensory spinal nervous tissue (e.g., nervous tissue from dorsal nerve roots) transmits pain signals, as well as other sensory signals and proprioceptive signals.
Corresponding dorsal and ventral nerve roots depart the spinal cord “separately.” Laterally from the spinal cord, the nervous tissue of the dorsal and ventral nerve roots are mixed, or intertwined. Accordingly, electrical stimulation intended to manage and control one condition (e.g., pain) can inadvertently interfere with nerve transmission pathways in adjacent nervous tissue (e.g., motor nerves).
Electrical energy is conventionally delivered through electrodes positioned on the dorsal column external to the dura layer surrounding a spinal cord. The electrodes are typically carried by a percutaneous lead, although a laminotomy lead also can be used. Percutaneous leads commonly have two or more electrodes and are positioned within an epidural space through the use of an insertion, or Touhy-like, needle. An example of an eight-electrode percutaneous lead is an OCTRODE® lead manufactured by Advanced Neuromodulation Systems, Inc. of Plano, Tex. Operationally, the insertion needle is passed through the skin, between the desired vertebrae, and into an epidural space located between a dural layer and the surrounding vertebrae. The stimulation lead is fed through the bore of the insertion needle and into the epidural space. Laminotomy leads generally have a wider, paddle-like shape, and are inserted via an incision rather than through a needle. For example, a small incision is made in the back of a patient to access the space between the dura and the surrounding vertebrae.
According to the “gate-control” theory of Melzak and Wall, (Science, 150,971-978, 1965), the suppression of pain sensations, accompanied by paresthesia, results from the activation of large cutaneous afferents (Aαβ fibers). Because these nerve fibers are part of the dorsal root (DR) fiber that ascends in the dorsal column (DC), paresthetic sensations can be evoked by both DC and DR stimulation.
The potential paresthesia coverage will strongly differ, however, depending on whether DC fibers or DR fibers are stimulated. When stimulating the DC fibers, the fibers corresponding to all dermatomes from the sacral ones up to the electrode level may be activated, thus resulting in broad paresthesia coverage. When stimulating DR fibers, however, the fibers will be activated in a limited number of rootlets close to the cathodal contact(s), thereby resulting in a paresthesia effect confined to one or two dermatomes at each body side.
There are several problems with existing Spinal Cord Stimulation (SCS) therapy techniques. One is the difficulty of obtaining a permanent optimal position of the lead(s), to cover the painful dermatomes with paresthesia. Another problem is the usually small range of stimulation amplitudes between the perception threshold (i.e., the threshold at which paresthesia is effected) and the discomfort threshold (i.e., the threshold at which the patient experiences pain or other discomfort), often preventing a complete coverage of the painful area by the paresthesia needed for maximum therapeutic effect (Holsheimer, Neurosurgery, 40, 5, 990-999, 1997).